Carbon nanotubes have a circular cylindrical structure composed of a rolled graphene sheet, and generally have a straw-shaped structure.
Further, carbon nanotubes are classified into single walled carbon nanotubes (SWCNT) formed from single layer tubes, double walled carbon nanotubes (DWCNT) having a laminated structure (two-layer structure) composed of two tubes having different diameters, and multi walled carbon nanotubes (MWCNT) having a laminated structure (multi-layer structure) composed of a plurality of tubes having different diameters, and much applied research is being conducted into harnessing the properties of these respective structures.
Among the above types of carbon nanotubes, SWCNT may adopt a structure having semiconductor properties depending on the way in which the graphene sheet is rolled, and because they are expected to exhibit superior mobility, applications of these SWCNT to thin-film transistors (TFT) holds considerable promise, and research into such applications is being actively pursued. For example, Non-Patent Documents 1 to 4 and the like disclose that TFTs using carbon nanotubes exhibit performance that matches or exceeds that of silicon.
In those cases where carbon nanotubes are used as one component of a channel semiconductor material, the TFT is produced by either dispersing one or several carbon nanotubes in the semiconductor material, or dispersing a multitude of carbon nanotubes in the semiconductor material.
In those cases where one or several carbon nanotubes are used, because the length of the carbon nanotubes is generally approximately 1 μm or less, microfabrication techniques are required during production of the TFT, and the so-called channel length between the source electrode and the drain electrode must be produced at the sub-micron level.
In contrast, in those cases where a multitude of carbon nanotubes are used, because a network of the carbon nanotubes is used as the channel, the length of the channel can be increased, and the channel can be produced relatively easily. One example of a method of producing a TFT by dispersing a multitude of carbon nanotubes in a solvent or the like is disclosed in Non-Patent Document 5.
Furthermore, because DWCNT and MWCNT exhibit high levels of electrical conductivity, applications of these types of carbon nanotubes to electrode materials, wiring materials, antistatic films and transparent electrodes holds considerable promise, and research into such applications is progressing.
In order to form a TFT in which a multitude of carbon nanotubes have been uniformly dispersed, the multitude of carbon nanotubes are first dispersed in a solvent or the like to form a dispersion, and this dispersion is then used to form the TFT. For example, Non-Patent Documents 6 to 9 and the like disclose methods of forming thin films composed of carbon nanotubes using this type of dispersion. By forming a thin film composed of carbon nanotubes by using a dispersion containing the carbon nanotubes, not only can elements, devices or product substrates be imparted with flexibility, but by applying the dispersion to hard materials such as glass, or other materials such as natural resins or plastics, flexibility can also be imparted to the overall element, device or product. Moreover, by using this type of dispersion, a typical coating process can be employed, meaning a production method that employs a coating process or printing process can be used to reduce the production costs of the element, device or product.